Compensating for drifts occurring during sleep times in access terminals

ABSTRACT

A method and apparatus are presented for compensating drifts in access terminals occurring during a sleep time. The method includes determining whether a sleep time exceeds a threshold, buffering time domain samples containing acquisition pilots and a paging channel, powering down RF circuitry in the access terminal after buffering samples, processing the samples to compensate for drift, and determining whether the access terminal was paged based upon the processed samples. The apparatus includes a digital front end, an FFT engine coupled to the digital front end, a symbol buffer coupled to the FFT engine, a processor coupled to the digital front end, FFT engine, and symbol buffer, and a memory coupled to the processor, the memory further comprising instructions for executing the method.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present Application for Patent claims priority to ProvisionalApplication No. 61/041,324 entitled “METHOD AND APPARATUS FOR HANDLINGDRIFTS DURING SLEEP FOR ACCESS TERMINALS” filed Apr. 1, 2008, andassigned to the assignee hereof and hereby expressly incorporated byreference herein.

FIELD OF DISCLOSURE

Embodiments of the disclosure generally relate to communications in awireless environment. More particularly, embodiments of the disclosurerelate to compensating drifts of components within a wireless accessterminal which occur during sleep times.

BACKGROUND

Wireless communication systems are widely deployed to provide variouscommunication services such as voice, video, packet data, messaging,broadcast, etc. These wireless systems may be multiple-access systemscapable of supporting multiple users by sharing the available systemresources (e.g., bandwidth and transmit power). Examples of suchmultiple-access systems include Code Division Multiple Access (CDMA)systems, Time Division Multiple Access (TDMA) systems, FrequencyDivision Multiple Access (FDMA) systems, 3GPP LTE systems, OrthogonalFDMA (OFDMA) systems, Single-Carrier FDMA (SC-FDMA) systems, etc.

In wireless communication systems access terminals (referred to asmobile stations, handsets, mobile devices, and/or user terminals)receive signals from fixed position access points (also referred to asbase stations, Node-B, cell sites or cells) that support communicationlinks or service within particular geographic regions adjacent to orsurrounding the access point. In order to aid in providing coverage,each cell may be sub-divided into multiple sectors, each correspondingto a smaller service area or geographic region. An array or series ofaccess points placed adjacent to each other can form a communicationsystem capable of servicing a number of system users, over a largerregion.

Generally, a wireless multiple-access communication system cansimultaneously support communication for multiple wireless accessterminals. Each access terminal may communicate with one or more accesspoints via transmissions on the forward and reverse links. The forwardlink (or downlink) refers to the communication link from the accesspoints to the terminals, and the reverse link (or uplink) refers to thecommunication link from the terminals to the access points. Thiscommunication link may be established via a single-in-single-out,multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.

Each access terminal can monitor a control channel that may be used toexchange messages between the access terminal and the access point. Thecontrol channel is used to transmit system/overhead messages, whereastraffic channels are typically used for substantive communication (e.g.,voice and data) to and from the access terminal. For example, thecontrol channel can be used to establish traffic channels, control powerlevels, and the like, as is known in the art.

Because the access terminals are typically battery operated, powerconservation is emphasized in the system design. Accordingly, accessterminals can enter into sleep modes and periodically awaken to monitorthe control channel for messages/paging directed to the access terminal.During sleep modes, component(s) within the access terminal mayexperience drifts. These drifts may be characterized as uncontrolledvariations in the performance of components in the Access Terminal. Forexample, an oscillator used as a frequency reference in the accessterminal may provide a clock signal which experiences time and/orfrequency variations. Component drift can adversely affect thefunctionality and/or performance of the access terminal. Moreover, thistiming/frequency drift may also affect the performance of other users inthe Uplink (UL) by violating the time/frequency orthogonality acrossusers.

Accordingly, it is desirable to compensate for component drift in orderto mitigate potentially adverse effects on the communication system.

SUMMARY

Exemplary embodiments are directed to systems and method forcompensating for drifts occurring during sleep times in accessterminals.

In one embodiment, a method for compensating drifts in access terminalsoccurring during a sleep time is presented. The method includesdetermining whether a sleep time exceeds a threshold, buffering timedomain samples containing acquisition pilots and a paging channel,powering down RF circuitry in the access terminal after bufferingsamples, processing the samples to compensate for drift, and determiningwhether the access terminal was paged based upon the processed samples.

In another embodiment, an apparatus for compensating drifts in accessterminals occurring during a sleep time is presented. The apparatusincludes a digital front end, an FFT engine coupled to the digital frontend, a symbol buffer coupled to the FFT engine, a processor coupled tothe digital front end, FFT engine, and symbol buffer, and a memorycoupled to the processor, the memory further comprising instructionswhich determines whether a sleep time exceeds a threshold, buffers timedomain samples containing acquisition pilots and a paging channel,powers down RF circuitry in the access terminal after buffering samples,processes the samples to compensate for drift, and determines whetherthe access terminal was paged based upon the processed samples.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are presented to aid in the description ofembodiments of the disclosure and are provided solely for illustrationof the embodiments and not limitation thereof.

FIG. 1 shows a top level diagram of an exemplary multiple accesswireless communications system.

FIG. 2 shows block diagrams of an exemplary access terminal and accesspoint within the wireless communications system.

FIG. 3 depicts a diagram of a format associated with an exemplary superframe structure.

FIG. 4 shows a block diagram of an exemplary hardware receiverarchitecture associated with an access terminal.

FIG. 5 shows a flow diagram of an exemplary process for acquiring timedomain samples and compensating drift within an access terminal.

FIG. 6 shows a flow diagram of an exemplary process for correcting timeand/or frequency drifts within the compensation process shown in FIG. 5.

DETAILED DESCRIPTION

Embodiments are disclosed in the following description and relateddrawings directed to specific embodiments of the disclosure. Alternateembodiments may be devised without departing from the scope of theinvention. Additionally, well-known elements will not be described indetail or will be omitted so as not to obscure the relevant details ofthe disclosure.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. Likewise, the term “embodiments ofthe invention” does not require that all embodiments of the inventioninclude the discussed feature, advantage or mode of operation.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of embodiments ofthe invention. As used herein, the singular forms “a”, “an” and “the”are intended to include the plural forms as well, unless the contextclearly indicates otherwise. It will be further understood that theterms “comprises”, “comprising,” “includes” and/or “including”, whenused herein, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof.

Further, many embodiments are described in terms of sequences of actionsto be performed by, for example, elements of a computing device. It willbe recognized that various actions described herein can be performed byspecific circuits (e.g., application specific integrated circuits(ASICs)), by program instructions being executed by one or moreprocessors, or by a combination of both. Additionally, these sequence ofactions described herein can be considered to be embodied entirelywithin any form of computer readable storage medium having storedtherein a corresponding set of computer instructions that upon executionwould cause an associated processor to perform the functionalitydescribed herein. Thus, the various aspects of the invention may beembodied in a number of different forms, all of which have beencontemplated to be within the scope of the claimed subject matter. Inaddition, for each of the embodiments described herein, thecorresponding form of any such embodiments may be described herein as,for example, “logic configured to” perform the described action.

The techniques described herein may be used for various wirelesscommunication networks such as Code Division Multiple Access (CDMA)networks, Time Division Multiple Access (TDMA) networks, FrequencyDivision Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA)networks, Single-Carrier FDMA (SC-FDMA) networks, etc. The terms“networks” and “systems” are often used interchangeably. A CDMA networkmay implement a radio technology such as Universal Terrestrial RadioAccess (UTRA), cdma2000, etc. UTRA includes Wideband-CDMA (W-CDMA) andLow Chip Rate (LCR). cdma2000 covers IS-2000, IS-95 and IS-856standards. A TDMA network may implement a radio technology such asGlobal System for Mobile Communications (GSM). An OFDMA network mayimplement a radio technology such as Evolved UTRA (E-UTRA), IEEE 802.11,IEEE 802.16, IEEE 802.20, Flash-OFDM®, etc. UTRA, E-UTRA, and GSM arepart of Universal Mobile Telecommunication System (UMTS). Long TermEvolution (LTE) is an upcoming release of UMTS that uses E-UTRA. UTRA,E-UTRA, GSM, UMTS and LTE are described in documents from anorganization named “3rd Generation Partnership Project” (3GPP). cdma2000is described in documents from an organization named “3rd GenerationPartnership Project 2” (3GPP2). These various radio technologies andstandards are known in the art. For clarity, certain aspects of thetechniques are described below for LTE, and LTE terminology is used inmuch of the description below. Moreover, the procedures described hereinmay be used in FD-LTE and TD-LTE systems.

Single carrier frequency division multiple access (SC-FDMA), whichutilizes single carrier modulation and frequency domain equalization, isa wireless technique which builds on OFDMA. SC-FDMA has similarperformance and essentially the same overall complexity as an OFDMAsystem. However, an SC-FDMA signal has the advantage of a lowerpeak-to-average power ratio (PAPR) because of its inherent singlecarrier structure. SC-FDMA has drawn great attention, especially in theuplink communications where lower PAPR greatly benefits the mobileterminal in terms of transmit power efficiency. It is currently aworking assumption for the uplink multiple access scheme in 3GPP LongTerm Evolution (LTE), or Evolved UTRA.

FIG. 1 shows a top level diagram of an exemplary multiple accesswireless communications system. The system may be a MIMO system that canemploy multiple (N_(T)) transmit antennas and multiple (N_(R)) receiveantennas for data transmission. A MIMO channel formed by the N_(T)transmit and N_(R) receive antennas may be decomposed into N_(S)independent channels, which are also referred to as spatial channels,where N_(S)≦min{N_(T), N_(R)}. Each of the N_(S) independent channelsmay correspond to a dimension. The MIMO system can provide improvedperformance (e.g., higher throughput and/or greater reliability) if theadditional dimensionalities created by the multiple transmit and receiveantennas are utilized.

A wireless system may be a time division duplex (TDD) and/or a frequencydivision duplex (FDD) system. In a TDD system, the forward and reverselink transmissions are on the same frequency region so that thereciprocity principle allows the estimation of the forward link channelfrom the reverse link channel. This enables the access point to extracttransmit beamforming gain on the forward link when multiple antennas areavailable at the access point.

Further referring to FIG. 1, an access point 100 (AP) may includemultiple antenna groups, one including antennas 104 and 106, anotherincluding antennas 108 and 110, and an additional including antennas 112and 114. In FIG. 1, only two antennas are shown for each antenna group,however, different numbers of antennas may be utilized for each antennagroup. Access terminal 116 (AT) is in communication with antennas 112and 114, where antennas 112 and 114 may transmit information to accessterminal 116 over forward link 120, and receive information from accessterminal 116 over reverse link 118. Access terminal 122 may be incommunication with antennas 106 and 108, where antennas 106 and 108transmit information to access terminal 122 over forward link 126, andreceive information from access terminal 122 over reverse link 124. In aFDD system, communication links 118, 120, 124 and 126 may use differentfrequency for communication. For example, forward link 120 may use adifferent frequency then that used by reverse link 118.

Each group of antennas and/or the area in which they are designed tocommunicate is often referred to as a sector of the access point. In theembodiment shown in FIG. 1, each antenna group may be designed tocommunicate to access terminals in a designated sector within the areascovered by access point 100.

In communication over forward links 120 and 126, the transmittingantennas of access point 100 may utilize beamforming in order to improvethe signal-to-noise ratio of forward links for the different accessterminals 116 and 124. Using beamforming to transmit to access terminalsscattered randomly throughout a coverage area may cause lessinterference to access terminals in neighboring cells than an accesspoint transmitting through a single antenna to all its access terminals.

FIG. 2 shows block diagrams of an exemplary access terminal 250 andaccess point 210 within the wireless communications system. In thisembodiment, the communication system may be a MIMO system 200, which caninclude the Access Point 210 and the Access Terminal 250. Downlink (DL)transmission occurs from Access Point to the Access Terminal. Uplink(UL) transmission occurs from Access Terminal to the Access Point. Atthe access point 210, traffic data for a number of data streams may beprovided from a data source 212 to a transmit (TX) data processor 214.Each data stream may be transmitted over a respective transmit antenna.TX data processor 214 may format, code, and interleave the traffic datafor each data stream based on a particular coding scheme selected forthat data stream to provide coded data.

The coded data for each data stream may be multiplexed with pilot datausing OFDM techniques. The pilot data is typically a known data patternthat is processed in a known manner and may be used at the receiversystem to estimate the channel response. The multiplexed pilot and codeddata for each data stream may then modulated (i.e., symbol mapped) basedon a particular modulation scheme (e.g., BPSK, QSPK, M-PSK, M-QAM, etc.)selected for that data stream to provide modulation symbols. The datarate, coding, and modulation for each data stream may be determined byinstructions performed by processor 230.

The modulation symbols for all data streams may then be provided to a TXMIMO processor 220, which may further process the modulation symbols(e.g., for OFDM). TX MIMO processor 220 can then provide N_(T)modulation symbol streams to N_(T) transmitters (TMTR) 222 a through 222t. In certain embodiments, TX MIMO processor 220 may apply beamformingweights to the symbols of the data streams and to the antenna from whichthe symbols are being transmitted. Each transmitter 222 may receive andprocesses a respective symbol stream to provide one or more analogsignals, and further conditions (e.g., amplifies, filters, and/orup-converts) the analog signals to provide a modulated signal suitablefor transmission over the MIMO channel. N_(T) modulated signals fromtransmitters 222 a through 222 t may be then transmitted from N_(T)antennas 224 a through 224 t, respectively.

At access terminal 250, the downlink (DL) signals from the Access Pointmay be received by N_(R) antennas 252 a through 252 r and the receivedsignal from each antenna 252 which may be provided to a respectivereceiver (RCVR) 254 a through 254 r. Each receiver 254 may condition(e.g., filters, amplifies, and down-converts) a respective receivedsignal, digitize the conditioned signal to provide samples, and canfurther process the samples to provide a corresponding “received” symbolstream.

An RX MIMO processor 260 may then receive and processes the N_(R)received symbol streams from N_(R) receivers 254 based on a particularreceiver processing technique to provide N_(T) “detected” symbolstreams. The RX data processor 261 may then demodulate, de-interleave,and decode each detected symbol stream to recover the traffic data forthe data stream. The processing by RX MIMO processor 260 iscomplementary to that performed by TX MIMO processor 220. The processingby RX data processor is complementary to that performed by TX dataprocessor 214 at access point 210.

Processor 270 may then formulate a reverse link message that maycomprise various types of information regarding the communication linkand/or the received data stream. The reverse link message may then beprocessed by a TX data processor 238, which also receives traffic datafor a number of data streams from a data source 236, modulated by a TxMIMO processor 280, conditioned by transmitters 254 a through 254 r, andtransmitted back to transmitter system 210. At transmitter system 210,the modulated signals from receiver system 250 are received by antennas224, conditioned by receivers 222, demodulated by a RX MIMO processor240, and processed by a RX data processor 242 to extract the reservelink message transmitted by the receiver system 250.

The transmission timeline for the forward and reverse links may bepartitioned into units of superframes. FIG. 3 depicts a diagram of aformat associated with an exemplary superframe 300 structure. Eachsuperframe may span a particular time duration, which may be fixed orconfigurable. In the embodiment shown in FIG. 3, the superframe preamblemay repeat approximately every 25 msec. On the forward link, asuperframe 300 may include a preamble followed by M physical layer (PHY)frames, where M may be any integer value. On the reverse link, eachsuperframe 310 may include M PHY frames, where the first PHY frame maybe extended by the length of the superframe preamble on the forward link(for example, as shown in FIG. 3, frame 0 may include 16 OFDM symbols).In the design shown in FIG. 3, each superframe includes 25 PHY frameswith indices of 0 through 24. Each PHY frame may carry traffic data,signaling, pilot, etc.

The superframe preamble 305 may include information to allow the accessterminal 250 to perform paging and acquisition operations. Informationfor quick paging may be provided over a paging channel such as, forexample, the Quick Page Channel (QPCH). Information for acquisition mayreside in the Time Division Multiplexed (TDM) pilots 1, 2 and 3. In oneembodiment, the superframe preamble may include eight OFDM symbols withindices of 0 through 7. The OFDM symbol 0 may comprises a ForwardPrimary Broadcast Control Channel (F-PBCCH) that carries information fordeployment-specific parameters. OFDM symbols 1 through 4 may compriseeither a Forward Secondary Broadcast Control Channel (F-SBCCH) or aForward Quick Paging Channel (F-QPCH). The F-SBCCH may carry informationfor sector-specific parameters. The F-QPCH may carry information usedfor quick paging.

The OFDM symbols 5, 6 and 7 may comprise time division multiplexed (TDM)pilots 1, 2 and 3, respectively, which may be used by terminals forinitial acquisition as described above. TDM pilot 1 may be used as aForward Acquisition Channel (F-ACQCH). A ForwardOther-Sector-Interference Channel (F-OSICH) may be sent in TDM pilots 2and 3. One would appreciate that the superframe preamble may also bedefined in other manners, and the paging may be performed using avariety of signals and channels, accordingly the format and channelstructure provided above is merely exemplary.

For example, in Long Term Evolution (LTE) systems, the equivalents forTDM pilots 1 and TDM pilots 2 may be Primary and SecondarySynchronization Signals (PSS, SSS), respectively. In other embodiments,signals such as the Primary Pilot Channel (PPICH), or the LTE equivalentCommon Reference Signal may be used in place of the synchronizationsignals for the search and/or pilot strength measurements. Moreover, inanother embodiment, the paging may be performed using a data channelsuch as, for example, the Primary Data Shared Channel (PDSCH).

In one embodiment, with respect to quick paging operations, when theaccess terminal sleeps, it should wake up periodically to read the QPCH.If the QPCH decode is successful (that is, the message successfullypasses a CRC test) and the terminal is paged, it should decode theFull-Page Channel to determine the paging details. The Full-Page channelmay be transmitted on regular PHY Frames using Hybrid ARQ (HARQ). Thetransmission may span 6 Frames, that are separated by approximately ˜5msec apart. A terminal with good SNR may decode the Full Page in 1Frame, whereas a terminal with poor SNR can take up to 6 Frames todecode Full Page. Hence, the total decode times for Full Page can be ashigh as approximately 30 msec. In general, decoding Full Page channelconsumes excess power and wastes battery life. It is for this reasonthat the QPCH channel was introduced, that is, in order to limit thenumber of times the access terminal 250 has to decode the Full Page, andthus enhance battery life.

In order to decode any received channel upon being initialized afterpower-up, the access terminal 250 should first perform acquisition. Whenthe access terminal 250 is initially powered up, it should determine thetiming and frequency offset of the Access Terminal with respect to theAccess Point in order to enable successful decoding of the DL channels.To determine these offsets, the access terminal performs what is definedherein as “acquisition.” The acquisition procedure can lock on to theTDM-1, 2, 3 pilot symbols, and thereby establishes correct timing andfrequency offsets. In other words, after acquisition, the accessterminal is capable of decoding other channels such as, for example, theQPCH channel, the DCH (data channel) etc.

In one embodiment, the superframe preamble may include eight OFDMsymbols with indices of 0 through 7. The OFDM symbol 0 may comprises aForward Primary Broadcast Control Channel (F-PBCCH) that carriesinformation for deployment-specific parameters. OFDM symbols 1 through 4may comprise either a Forward Secondary Broadcast Control Channel(F-SBCCH) or a Forward Quick Paging Channel (F-QPCH). The F-SBCCH maycarry information for sector-specific parameters. The F-QPCH may carryinformation used for quick paging described above.

FIG. 4 shows a block diagram of an exemplary hardware architecture for areceiver 400 associated with an access terminal. The receiver may becomprised of a series of signal processing functional blocks, includingFront End 405, Sample Server 410, FFT Engine 415, Symbol Buffer 420,Demodulator 425, and Decoder 430. The signal processing blocks may becontrolled by a processor 440, which interfaces to the signal processingblock over a Hardware/Firmware (HW/FW) interface 435. The processor 440,which may be at least one micro-processor, a micro-controller, a DigitalSignal Processor (DSP), etc., or any combination thereof, may includeonboard and/or external memory 445 which stores program code and anyassociated parameters and data. The program code may be realized in theform of software, firmware, or any combination thereof.

The received baseband I & Q signal time domain samples, obtained bydigitizing the received signal via an Analog-to-Digital Converter (ADC),may be fed to the Digital Front End Block 405. The Digital Front Endblock performs signal conditioning such as digital AGC and filtering.Note that a modem typically also has an analog Front End Block that ispart of the RF circuitry (not shown). The RF circuitry includes analogcomponents like analog AGC, mixer, analog filters etc and operate on thereceive signal before it is fed to the ADC.

The time domain samples may be passed on to the sample server 410 wherethey are buffered prior be converted into the frequency domain. The timedomain samples may be converted to frequency domain symbols by using anFFT Engine 415. The symbols may then be buffered in Symbol Buffer 420.The symbols may be demodulated into soft information bits by demodulator425, and subsequently decoded in decoder 430. The demodulator 425 mayhave a MIMO receiver such as an MMSE receiver, followed by a LogLikelihood Ratio (LLR) computing engine. The decoder 430 may include aViterbi decoder, a Turbo decoder and/or a LDPC decoder.

As mentioned previously, the access terminal's sleep time may beincreased during periods of terminal inactivity in order to save batterylife. However, significant time and/or frequency drift can arise due tothe sleep clock drifting. As an example, 2 ppm sleep clock drift in a 20MHz system can lead to a timing drift of ˜20 us, over a 10 sec sleepduration. In one embodiment such as in the LTE or UMB or 802.20standards, an OFDM symbol can be ˜100 us long, in which case the timingdrift is approximately ⅕ of an OFDM symbol period. In anotherembodiments, such as 802.11 WLAN standards, an OFDM symbol can be 4 uslong, in which case the timing drift can span approximately 5 OFDMsymbols. Furthermore, using lower cost crystal oscillators (XO) that mayhave a higher drift specification in ppm (e.g.,: 50-100 ppm) may resultin much larger time/frequency drifts over this time-scale. Due to suchtime and/or frequency drifts, the access terminal may not be able todecode a paging channel (e.g., QPCH) upon wake-up, since typically theQPCH channel has a higher spectral efficiency and may be susceptible todistortion introduced by time/frequency drifts. As a result, the accessterminal does not know whether it is being paged or not. This leads tothe terminal attempting to decode the Full Page channel, causing it tobe awake for up to 30 msec, as explained above. Typically, the terminalis better positioned to decode the Full Page Channel even in thepresence of timing/frequency drift since the information is encodedacross 6 Frames spread over 30 msec, leading to a very small spectralefficiency (smaller than the spectral efficiency of QPCH channel).However, if the time drift is significant fraction of the OFDM symbol(say >25%) , or if the frequency drift is a significant fraction of theOFDM tone-spacing (say 25%), then it is highly likely that the Full Pagedecode might fail. This leads to the terminal re-running acquisitionafter some time-out period to obtain a fresh time/frequency offset, andthen employing them to decode a full-page.

All the above activities can lead to expending significant battery powerand reducing the standby time. To mitigate this effect, conventionalaccess terminals might reduce sleep times to shorten clock drifting,which again impacts efficiency. Alternately, access terminals may employa more expensive LO that experiences a smaller ppm sleep drift, whichincreases terminal cost.

Embodiments of the disclosure improve the standby time by compensatingfor the time and/or frequency drift of the sleep clock, with minimalpower consumption. This compensation may be performed as follows. Whensleep times are sufficiently long and exceed a predetermined period oftime, the access terminal may awaken early to buffer samples that alsocontain the superframe preamble, at some unknown time-offset in thebuffered samples. For example, if the terminal is using a 5 ppm clockand sleeps for 10 seconds, it may wake up ˜50 us earlier to buffersamples, assuming that the worse-case clock drift of ˜50 us. Since theactual ppm offset may be smaller than 5 ppm at any given time instant,the buffered samples will in reality contain the superframe preamble atsome unknown offset of up to 50 us. The buffered samples, specificallythe TDM pilots, are then analyzed and processed in order to determineany time and/or frequency offsets values. The buffered samples may thenbe corrected for the time/frequency offset values prior to performing aquick paging operation.

This buffering may be performed by the processor 440 executingacquisition algorithm 450, by storing the samples in the Tightly CoupledMemory (TCM) of the processor. Alternately, the processor may controlthe FFT engine 415 and place it in a by-pass mode so that time domainsamples can be buffered in the symbol buffer 420. The drift compensationalgorithm 455 may compute correction factors from the drift offsetvalues derived the buffered samples, and apply the values to thebuffered samples to compensate for the drift. Details of the acquisitionand drift compensation algorithms are presented below in the descriptionof FIGS. 5 and 6.

FIG. 5 shows a flow diagram of an exemplary process 500 for acquiringtime domain samples and compensating drift within an access terminal250.

After going to sleep, the access terminal 250 may wake up at timeduration=Sleep Cycle−deltaT, where deltaT is the sleep-drift. In oneexample, assuming a 5 ppm maximum clock drift and 10 sec sleep duration,deltaT can be 50 us. The processor can then buffer the time domainsamples upon wakeup (Block 520). In some embodiments, the time-domainssamples may be part of the superframe preamble.

The storage location of the time domain samples can accomplished using avariety of different memory locations available in the access terminal250, such as, for example, a symbol-RAM in the symbol buffer 420, aTightly Coupled Memory (TCM) memory of the process, etc. The processor440 may determine where the symbols are buffered by controlling any ofthe appropriate signal processing blocks via the HW/FW interface 435.For example, in one embodiment, the processor 440 may place the FFTengine 415 in a bypass mode (in order to avoid transforming the acquiredsamples into the frequency domain) to move samples from sample-server410 to symbol RAM or TCM for buffering.

After buffering the time domain samples, the processor 440 may powerdown the RF circuitry to save power (Block 525). The processor may thenbegin pure digital processing of superframe preamble to determine andcorrect drift (Block 530). The details of this processing are presentedin FIG. 6.

Next, the access terminal 250 may perform regular demodulationprocessing of the paging channel using the corrected buffered timedomain samples to determine if the access terminal 250 has been paged(Block 535). If the paging channel decode results in a successful CRCand the access terminal 250 detects a page (Block 540), then it mayproceed to decode the full page of the next superframe (Block 545).Otherwise, if the paging channel decode results in a successful CRC anda page is not detected in Block 540, the access terminal 250 may reenterthe sleep state (Block 550).

The processor 440 in access terminal 250 may also choose to do some orall parts of paging channel demodulation and time/frequency correctionoffline in firmware (FW), depending upon its hardware capability. Fordecoding a full-page channel, the terminal can use the already computedtime/frequency offset to wake up at the correct time, and also apply thecorrect frequency offset to the phase lock loop (PLL), digital frequencycorrection block or the Voltage Controlled Temperature CompensatedCrystal Oscillator (VCTCXO).

Accordingly, the wakeup time according to the above-described methodshown in FIG. 5 is significantly smaller than conventional wakeupprocesses, thereby leading to a substantial increase in standby time.This approach also improves the performance of the paging channeldemodulation and decoding, in the presence of timing/frequency drifts,thereby leading to a lower probability of reading subsequent pages, suchas, for example, full-pages and/or full-page failures, thus conservingbattery power.

Note that if the sleep-drift (deltaT) is less than some Threshold (e.g.,less than 1/20^(th) of the OFDM symbol duration), then the AccessTerminal may choose to wake up at time duration=Sleep Cycle, and proceedwith a regular demodulation of the paging channel to determine if it waspaged. In other words, all the above process for time/frequency offsetestimation and correction, including buffering of time domain samplesmay be skipped. This is done to further reduce power consumption.

FIG. 6 shows a flow diagram of an exemplary process for correcting timeand/or frequency drifts within the compensation process 530 shown inFIG. 5.

After buffering the samples in Block 520 shown in FIG. 5, the processor440 may direct access terminal 250 to first processes TDM-1,2,3 pilotsamples in a cold-start acquisition mode in order to compute the timeand/or frequency offset(s) of the clock (Blocks 605 and 610). Next, theprocessor 440 may compensate for the offset(s) by applying a time and/orfrequency correction on the buffered samples corresponding to the pagingchannel (Block 615). The time correction may be applied by changing thestarting location of the 1st OFDM symbol of paging channel, according tothe temporal offset detected. The frequency correction may be applied byusing a time-domain phase ramp corresponding to the estimated frequencyoffset. For example, if the frequency offset is denoted as “f”, thephase ramp is given as theta(t)=exp(j*2*π*f*t), where j=sqrt(−1), and tis the time. This phase ramp is point-wise multiplied by the receivedsamples to obtain a frequency corrected received samples.

Those of skill in the art will appreciate that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof

Further, those of skill in the art will appreciate that the variousillustrative logical blocks, modules, circuits, and algorithm stepsdescribed in connection with the embodiments disclosed herein may beimplemented as electronic hardware, computer software, or combinationsof both. To clearly illustrate this interchangeability of hardware andsoftware, various illustrative components, blocks, modules, circuits,and steps have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the present invention.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by acomputer. By way of example, and not limitation, such computer-readablemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium that can be used to carry or store desired program code inthe form of instructions or data structures and that can be accessed bya computer. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and Blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

Accordingly, an embodiment of the invention can include a computerreadable media embodying a method for compensating drifts occurringduring sleep times in access terminals. Accordingly, the invention isnot limited to illustrated examples and any means for performing thefunctionality described herein are included in embodiments of theinvention.

While the foregoing disclosure shows illustrative embodiments of theinvention, it should be noted that various changes and modificationscould be made herein without departing from the scope of the inventionas defined by the appended claims. The functions, steps and/or actionsof the method claims in accordance with the embodiments of the inventiondescribed herein need not be performed in any particular order.Furthermore, although elements of the invention may be described orclaimed in the singular, the plural is contemplated unless limitation tothe singular is explicitly stated.

1. A method for compensating drifts occurring during a sleep time in anaccess terminal, comprising: determining whether a sleep time exceeds athreshold; buffering time domain samples from at least one acquisitionor paging channel; powering down RF circuitry in the access terminalafter buffering the time domain samples; processing the time domainsamples to compensate for drift; and determining whether the accessterminal was paged based upon the processed time domain samples.
 2. Themethod according to claim 1, wherein the time domain samples furthercomprise acquisition pilots.
 3. The method according to claim 2, whereinthe paging channel comprises the Quick Paging CHannel (QPCH).
 4. Themethod according to claim 2, wherein determining whether the accessterminal was paged further comprises: demodulating the paging channel;and determining the terminal was paged based upon the successfullydecoding the paging channel.
 5. The method according to claim 4, furthercomprising: decoding a subsequent page when the page based upon thepaging channel is detected.
 6. The method according to claim 2, whereinthe processing further comprises: processing TDM 1, 2, 3 samples in acold start acquisition mode; determining at least one of time andfrequency offset correction values; and applying the at least one timeand frequency offset correction values to the buffered paging channelsamples.
 7. The method according to claim 6, further comprising:applying the time correction by determining the starting location of thefirst OFDM symbol of the paging channel.
 8. The method according toclaim 6, further comprising: applying the frequency correction using atime-domain phase ramp corresponding to the determined frequency offsetcorrection value.
 9. The method according to claim 1, wherein the timedomain samples are buffered in a buffer space which includes at leastone of a symbol-RAM, a TCM memory in the processor, and a sample-server.10. The method according to claim 9, further comprising: operating anFFT engine in a bypass mode to transfer the time domain samples from thesample server to the symbol-RAM.
 11. The method according to claim 2,further comprising: determining that the sleep time did not exceed thethreshold; and demodulating the paging channel without buffering andprocessing of the time domain samples.
 12. The method according to claim11, further comprising; placing the AT in a sleep state upon detectingthat the terminal was not paged after demodulating the paging channel.13. An apparatus for compensating drifts occurring during a sleep timein an access terminal, comprising: a digital front end; an FFT enginecoupled to the digital front end; a symbol buffer coupled to the FFTengine; a processor coupled to the digital front end, FFT engine, andthe symbol buffer; a memory coupled to the processor, the memory furthercomprising instructions which determines whether a sleep time exceeds athreshold, buffers time domain samples from at least one acquisition orpaging channel, powers down RF circuitry in the access terminal afterbuffering the time domain samples, processes the time domain samples tocompensate for drift, and determines whether the access terminal waspaged based upon the processed time domain samples.
 14. The apparatusaccording to claim 13, wherein the time domain samples further compriseacquisition pilots.
 15. The apparatus according to claim 14, wherein theinstructions which determine whether the access terminal was pagedcomprise further instruction which: demodulates the paging channel; anddetermines whether the terminal was paged based upon the successfuldemodulating of the paging channel.
 16. The apparatus according to claim15, wherein the memory comprises further instructions which demodulate asubsequent page when the page based upon the paging channel is detected.17. The apparatus according to claim 16, wherein the memory comprisesfurther instructions which process TDM 1, 2, 3 samples in a cold startacquisition mode, determines at least one of time and frequency offsetcorrection values, and applies the at least one time and frequencyoffset correction values to the buffered paging channel samples.
 18. Theapparatus according to claim 17, wherein the memory comprises furtherinstructions which applies the time correction by determining thestarting location of the first OFDM symbol of the paging channel. 19.The apparatus according to claim 17, wherein the memory comprisesfurther instructions which applies the frequency correction using atime-domain phase ramp corresponding to the determined frequency offsetcorrection value.
 20. An apparatus for compensating drifts occurringduring a sleep time in an access terminal, comprising: means fordetermining whether a sleep time exceeds a threshold; means forbuffering time domain samples from at least one acquisition or pagingchannel; means for powering down RF circuitry in the access terminalafter buffering the time domain samples; means for processing the timedomain samples to compensate for drift; and means for determiningwhether the access terminal was paged based upon the processed timedomain samples.
 21. The apparatus according to claim 20, wherein thetime domain samples further comprise acquisition pilots.
 22. Theapparatus according to claim 21, wherein the paging channel comprisesthe Quick Paging CHannel (QPCH).
 23. The apparatus according to claim21, further comprising: means for demodulating the paging channel; andmeans for determining whether the terminal was paged based upon thesuccessful decoding of the paging channel.
 24. The apparatus accordingto claim 23, further comprising; means for demodulating a subsequentpage when the page based upon the paging channel is detected.
 25. Theapparatus according to claim 22, further comprising: means forprocessing TDM 1, 2, 3 samples in a cold start acquisition mode; meansfor determining at least one of time and frequency offset correctionvalues; and means for applying the at least one time and frequencyoffset correction values to the buffered paging channel samples.
 26. Acomputer readable media embodying logic for compensating driftsoccurring during a sleep time in an access terminal, the logicconfigured to perform a method comprising: determining whether a sleeptime exceeds a threshold; buffering time domain samples from at leastone acquisition or paging channel; powering down RF circuitry in theaccess terminal after buffering the time domain samples; processing thetime domain samples to compensate for drift; and determining whether theaccess terminal was paged based upon the processed time domain samples.27. The computer readable media according to claim 26, wherein the timedomain samples further comprise acquisition pilots.
 28. The computerreadable media according to claim 27, wherein the paging channelcomprises the Quick Paging CHannel (QPCH).
 29. The computer readablemedia according to claim 27, comprising additional logic to perform themethod further comprising: demodulating the paging channel; anddetermining whether the terminal was paged based upon the successfuldemodulating of the paging channel.
 30. The computer readable mediaaccording to claim 29, comprising additional logic to perform the methodfurther comprising: demodulating a subsequent page when the page basedupon the paging channel is detected.
 31. The computer readable mediaaccording to claim 28, comprising additional logic to perform the methodfurther comprising: processing TDM 1, 2, 3 samples in a cold startacquisition mode; determining at least one of time and frequency offsetcorrection values; and applying the at least one time and frequencyoffset correction values to the buffered paging channel samples.
 32. Amethod for compensating drifts occurring during a sleep time in anaccess terminal, comprising: determining whether a sleep time exceeds athreshold; buffering time domain samples containing acquisition pilotsand a paging channel; and processing the time domain samples tocompensate for drift.
 33. The method according to claim 32, furthercomprising: powering down RF circuitry in the access terminal afterbuffering the time domain samples.
 34. The method according to claim 32,further comprising: determining whether the access terminal was pagedbased upon the time domain processed samples.
 35. The method accordingto claim 32, wherein the processing further comprises: processingPrimary and Secondary Synchronization Signal (PSS and SSS) samples in acold start acquisition mode; determining at least one of time andfrequency offset correction values; and applying the at least one timeand frequency offset correction values to the buffered paging samples.36. The method according to claim 34, wherein the page is sent on a datachannel.
 37. An apparatus for compensating drifts occurring during asleep time in an access terminal, comprising: means for determiningwhether a sleep time exceeds a threshold; means for buffering timedomain samples containing acquisition pilots and a paging channel; andmeans for processing the time domain samples to compensate for drift.38. The apparatus according to claim 37, further comprising: poweringdown RF circuitry in the access terminal after buffering the time domainsamples.
 39. The apparatus according to claim 37, further comprising:determining whether the access terminal was paged based upon theprocessed time domain samples.
 40. The apparatus according to claim 37,wherein the processing further comprises: processing Primary andSecondary Synchronization Signal (PSS and SSS) samples in a cold startacquisition mode; determining at least one of time and frequency offsetcorrection values; and applying the at least one time and frequencyoffset correction values to the buffered paging samples.
 41. Theapparatus according to claim 39, wherein the page is sent on a datachannel.
 42. An apparatus for compensating drifts occurring during asleep time in an access terminal, comprising: a digital front end; anFFT engine coupled to the digital front end; a symbol buffer coupled tothe FFT engine; a processor coupled to the digital front end, FFTengine, and the symbol buffer; a memory coupled to the processor, thememory further comprising instructions which determines whether a sleeptime exceeds a threshold, buffers time domain samples containingacquisition pilots and a paging channel, and processes the time domainsamples to compensate for drift.
 43. The apparatus according to claim42, wherein the memory further comprising instructions which: power downRF circuitry in the access terminal after buffering the time domainsamples.
 44. The apparatus according to claim 42, wherein the memoryfurther comprising instructions which: determine whether the accessterminal was paged based upon the processed time domain samples.
 45. Theapparatus according to claim 42, wherein the memory further comprisinginstructions which: process Primary and Secondary Synchronization Signal(PSS and SSS) samples in a cold start acquisition mode; determine atleast one of time and frequency offset correction values; and apply theat least one time and frequency offset correction values to the bufferedpaging samples.
 46. The apparatus according to claim 44, wherein thepage is sent on a data channel.
 47. A computer readable media embodyinglogic for compensating drifts occurring during a sleep time in an accessterminal, the logic configured to perform a method comprising:determining whether a sleep time exceeds a threshold; buffering timedomain samples containing acquisition pilots and a paging channel; andprocessing the time domain samples to compensate for drift.